Generally, a vehicle door has a structure including two or more panels connected to each other for weight reduction and rigidity maintenance.
For example, FIG. 1 shows sectional views of a hemming structure in the related art and a problem of a conventional door. As shown in FIG. 1, the hemming structure of the conventional door includes an inner panel 10 disposed at the inner side of a vehicle body, and an outer panel 20 disposed at the outer side of the vehicle body. At a junction area between the inner panel 10 and the outer panel 20, a flange 21 protruding more than the inner panel 10 by a predetermined length is disposed at an end of the outer panel 20, and a hemming structure is formed by being bent such that the flange 21 is brought into contact with an opposite surface of the inner panel 10 or a surface facing the interior of the vehicle.
Further, a sealer 30 is applied onto the junction area between the inner panel 10 and the outer panel 20 to prevent penetration of moisture and foreign matter. The sealer 30 has an elongation property of about 10% and an impact property of about 35 N/mm, and is cured at a temperature greater than room temperature, preferably at the atmospheric temperature range of the painting process. The sealer 30 is cured as it passes through an oven during the painting process and maintains the impact property while increasing the bonding force between the inner and outer panels.
Meanwhile, in recent years, in order to improve the physical properties of a door for weight reduction and rigidity maintenance, a hybrid-type door, in which the outer panel is made of aluminum alloy material and the inner panel is made of iron steel material, has been proposed and used.
In the related art, the door of the hybrid type may be manufactured by hemming process as shown in {circle around (1)} of FIG. 1, and then it may pass through an oven during the painting process. As shown in {circle around (2)} of FIG. 1, due to the difference in thermal expansion coefficient between the inner panel 10 of iron steel and the outer panel 20 of the aluminum alloy, the outer panel 20 of the aluminum alloy may be expanded greater than the inner panel 10 of the iron steel. Then, a sealer is cured as shown in {circle around (3)} of FIG. 1 with the outer panel 20 expanded more than the inner panel 10. During the process of cooling after the painting process with the outer panel 20 and the inner panel 10 bonded together, the outer panel 20 of the aluminum alloy may contact greater than the inner panel 10 of steel as shown in {circle around (4)} of FIG. 1, thereby causing distortion.
Meanwhile, if the elongation of the sealer used in the hybrid-type door is improved, distortion caused by the difference in thermal expansion coefficient between different materials may be prevented, but a sealer with high elongation may not be used because of its low rigidity and bonding performance.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.